This invention relates in general to systems for measuring dopants, and in particular a system using difference frequency imaging and spectroscopy to measure dopants or other atomic characteristics using an alternating current scanning tunneling microscope (xe2x80x9cACSTMxe2x80x9d).
Scanning probe microscopes are extremely important for characterizing semiconductors with very high spatial resolution. According to the 1999 International Technology Roadmap for Semiconductors (1), there was already an unmet critical need to be able to determine 2-D dopant profiles with 3 nm resolution in 1999, and 1 nm spatial resolution will be needed by 2008. Much of the recent work in this field has focused on the development of the scanning capacitance microscope (SCM). See, for example:
(1) Capacitance-Voltage Measurement and Modeling on a Nanometer Scale by Scanning C-V Microscopy, by Y Huang and C. C. Williams; Journal of Vacuum Science and Technology B 12, 369 (1994)
(2) Quantitative Two-Dimensional Dopant Profile Measurement and Inverse Modeling by Scanning Capacitance Microscopy by Y Huang, C. C. Williams, and J. Slinkman; Applied Physics Letters 66, 344 (1995)
(3) Scanning Capacitance Microscopy and Spectroscopy Applied to Local Charge Modifications and Characterization of Nitride-Oxide-Silicon Heterostructures by M. Dreyer and R. Wiesendanger; Applied Physics A 61, 357 (1995)
(4) Scanning Capacitance Microscopy Measurements and Modeling: Progress Towards Dopant Profiling of Silicon, by J. J. Kopanski, J. F. Marchiando, and J. R. Lowney; Journal of Vacuum Science and Technology B 14, 242 (1996)
(5) Contrast Reversal in Scanning Capacitance Microscopy Imaging, by R. Stephenson, A. Verhulst, P. DeWolf, M. Caymax, and W. Vandervorst; Applied Physics Letters 73, 2597 (1998)
(6) Scanning Capacitance Microscope Methodology for Quantitative Analysis of p-n Junctions, by V. V. Zavyalov, J. S. McMurray, and C. C. Williams; Journal of Applied Physics 85, 7774 (1999)
(7) pn-Junction Delineation in Si Devices Using Scanning Capacitance Spectroscopy, by H. Edwards, V. A. Ukraintsev, R. San Martin, F. S. Johnson, P. Menz, S. Walsh, S. Ashburn, K. S. Wills, K. Harvey, and M. -C. Chang; Journal of Applied Physics 87, 1485 (2000)
These instruments have shown high sensitivity towards dopant density and type, and have accurately imaged devices on semiconductor surfaces with resolution as high as 10 nm. However, the lateral resolution when using capacitance detection is limited by the probe tip geometry and dopant level. Improving spatial resolution requires the development of new scanning probe techniques.
As one type of scanning capacitance microscope, a two-frequency mixing strategy designed to image p-n junctions using a microwave frequency compatible atomic force microscope (AFM) has been reported by J. Schmidt, D. H. Rapoport, G. Behme, and H. -J. Frohlich, J. Appl. Phys., 86, 7094 (1999). These particular AFM experiments used the sum and third harmonic frequencies as nonlinear mixing product signals. It was found that the sum frequency signal and the third harmonic signal are proportional to dC/dV and d2C/dV2, respectively, where C is the capacitance and V the voltage across the AFM tip and the sample.
None of the above systems is entirely satisfactory. It is therefore desirable to provide an instrument for measuring dopants with improved capabilities.
This invention is based on the observation that the above-described mixing strategy reported by J. Schmidt referenced above can be improved as follows. By measuring at a frequency substantially equal to the difference between the frequencies of two alternating current (AC) signals, or a multiple thereof, applied to a doped semiconductor material, it is possible for dopants to be detected at a much lower frequency than the frequencies of the AC signals applied. As used here-in-below, the term xe2x80x9cdifference frequencyxe2x80x9d refers to the difference between the frequencies of two alternating current (AC) signals, or a multiple thereof. This has the advantage of detecting signals at frequencies much below the microwave range so that the detection instrument can be much simplified compared to that employed by J. Schmidt referenced above.
Applicants discovered that the signal measured at the difference frequency depends upon the magnitude of the direct current (DC) bias voltage applied between the STM tip and the sample. Thus, preferably, the DC bias voltage is varied to optimize the difference frequency signal that is to be detected before the sample is measured at such DC bias voltage. For some applications, this means tuning the DC bias voltage until the amplitude of the difference frequency signals is at a maximum. For other applications, this may mean tuning the DC bias voltage until the best contrast is achieved between the measurement of two types of dopants. Still other optimization schemes are possible.
Applicants recognized that the difference frequency signal also depends upon the frequencies of the two or more AC signals applied to the sample. In other words, such amplitudes can be optimized by tuning the frequencies of the two or more AC signals applied to the sample. Therefore, preferably, the frequencies of the two or more AC signals applied to the sample can be swept in frequency either upwards or downwards until the amplitude of the difference frequency signal is optimized in one of a variety of ways as described herein.